[AC 1536] Breakdown The SubscriptionUpdate.cs into multiple files (#3356)

* Move sub-subscription classes to a separate files

* Refactor the sub-class to a separate files

* format whitespace

* remove directive that is unnecessary

* Remove the baseSeat class

src/Core/Models/Business/SeatSubscriptionUpdate.cs

@@ -0,0 +1,49 @@
+using Bit.Core.Entities;
+using Stripe;
+
+namespace Bit.Core.Models.Business;
+
+public class SeatSubscriptionUpdate : SubscriptionUpdate
+{
+    private readonly int _previousSeats;
+    private readonly StaticStore.Plan _plan;
+    private readonly long? _additionalSeats;
+    protected override List<string> PlanIds => new() { _plan.PasswordManager.StripeSeatPlanId };
+    public SeatSubscriptionUpdate(Organization organization, StaticStore.Plan plan, long? additionalSeats)
+    {
+        _plan = plan;
+        _additionalSeats = additionalSeats;
+        _previousSeats = organization.Seats.GetValueOrDefault();
+    }
+
+    public override List<SubscriptionItemOptions> UpgradeItemsOptions(Subscription subscription)
+    {
+        var item = SubscriptionItem(subscription, PlanIds.Single());
+        return new()
+        {
+            new SubscriptionItemOptions
+            {
+                Id = item?.Id,
+                Plan = PlanIds.Single(),
+                Quantity = _additionalSeats,
+                Deleted = (item?.Id != null && _additionalSeats == 0) ? true : (bool?)null,
+            }
+        };
+    }
+
+    public override List<SubscriptionItemOptions> RevertItemsOptions(Subscription subscription)
+    {
+
+        var item = SubscriptionItem(subscription, PlanIds.Single());
+        return new()
+        {
+            new SubscriptionItemOptions
+            {
+                Id = item?.Id,
+                Plan = PlanIds.Single(),
+                Quantity = _previousSeats,
+                Deleted = _previousSeats == 0 ? true : (bool?)null,
+            }
+        };
+    }
+}